[rom,refactor] Behavior of getting SPX key when SPX is disabled

### Description

The `sigverify_spx_key_get` is the function ROM uses to get the SPX public key. When SPX is disabled, the function returns OK while the key pointer is set to NULL.

https://github.com/lowRISC/opentitan/blob/605da270ddcf70b2af5573e9524f9613bc89383d/sw/device/silicon_creator/rom/sigverify_keys_spx.c#L15-L38

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Although this branch is currently unused in ROM, for a clearer API design, I would like to suggest returning an error when the SPX is disabled instead, so the function always returns a valid non-null key pointer when the status code is OK.

Start by reading sigverify_keys_spx.c, especially sigverify_spx_key_get and the linked lines, to understand the disabled-SPX branch and its status return. Confirm the API behavior for disabled SPX and verify that an OK result always corresponds to a non-null key pointer.

Written by the indexing model from the issue text.